In recent years, electric vehicles driven by electric motors are gaining popularity. An electric vehicle is equipped with a motor that is capable of generating drive torque for driving the vehicle, and includes an inverter that generates three-phase alternating current to be supplied to the motor and a control unit that controls the inverter. When performing control of the motor, it is important to increase the efficiency with which the motor is driven in order to reduce electric energy consumption of the motor as much as possible.
FIG. 10 illustrates a correlation between a rotation speed of a conventional motor, torque generated by the motor, and energy efficiency of the motor. As can be seen from FIG. 10, the energy efficiency of the motor changes depending upon both the rotation speed of the motor and the torque generated by the motor. In other words, even when the rotation speed of the motor does not change, the energy efficiency of the motor changes when the torque generated by the motor changes. In relation to this, conventional technology discloses a motor drive system that drives a motor intermittently at a drive point realizing high energy efficiency, thereby realizing a reduction in average energy loss of the motor and an increase in overall energy efficiency of the motor (e.g., Patent Literature 1).
FIG. 11 is a timing chart indicating a chronological change in torque generated by the motor when the motor drive technology disclosed in Patent Literature 1 is applied.
As can be seen from FIG. 11, within a period of a powering state where drive torque needs to be generated by the electric motor, a control unit sets, in alternation, a first drive period and a second drive period. The first drive period is a period during which the electric motor is driven according to a first drive torque command value that indicates a smaller torque value than a demanded torque value. On the other hand, the second drive period is a period during which the electric motor is driven according to a second drive torque command value that indicates a larger torque value than the demanded torque value. Here, the demanded torque value is a torque value that is determined according to the direction of an accelerator. By the first drive period and the second drive period being set in alternation during the period of the powering state as described above, the electric motor alternates between a state where three-phase alternating current is supplied thereto and a state where three-phase alternating current is not supplied thereto. Accordingly, torque is generated intermittently by the electric motor. Further, an average value of the first drive torque and the second drive torque generated by the electric motor during the period of the powering state is within a predetermined error range of the demanded torque value. Here, the energy efficiency of the electric motor, when driven according to the second drive torque command value, is set so as to indicate a higher value than the energy efficiency of the electric motor, when driven according to the demanded torque value.
By driving the electric motor in such a manner, the overall energy efficiency of the electric motor, when averaging the first drive torque and the second drive torque during the period of the powering state, is higher than the energy efficiency of the electric motor when the electric motor continuously outputs the demanded torque in accordance with the direction of the accelerator. As such, by setting two drive torque command values as described above, and further, by changing the torque generated by the electric motor by performing alternate switching between the two drive torque command values as illustrated in FIG. 11, the overall energy efficiency of the electric motor during the period of the powering state is improved while it is ensured that the torque generated by the electric motor equals the demanded torque without excess or deficiency.
In the meantime, electric vehicles are not only expected to reduce electrical power consumption, but also expected to provide, to its passengers, a comfortable ride. In relation to this, Patent Literature 1 discloses a technology of setting the first drive period and the second drive period during the period of the powering state such that a frequency at which the alternate switching between the first torque command value and the second torque command value is performed during the period of the powering state is sufficiently high compared to a natural frequency of a drive system. More specifically, according to Patent Literature 1, it is desirable that the frequency for alternate switching between the two torque command values be set to, for instance, a frequency around 100 Hz or higher. Commonly, a natural frequency of a drive system of an electric vehicle is around several Hz. As such, by setting the frequency for alternate switching between the two torque command values to around 100 Hz or higher, vibration of a chassis of an electric vehicle, etc., that takes place as a result of torque generated by a motor changing can be suppressed. Consequently, a comfortable ride can be realized. Note that here, the term “drive system” refers to an entirety of a device for transmitting torque generated by a motor to wheels of an electric vehicle.